Mark Zuckerberg said the search engine is "one of the coolest things we have done in a while"

Zuckerberg, 28, stressed that the new product was "very, very different" to web search engines offered by other tech giants such as Google because it only looks for data on Facebook itself.

It will allow users to search for people, photos, places and other content that has already been shared on the social network.

The announcement came after fevered speculation over what would be unveiled at the "come see what we're building" presentation.

Some reports had suggested Facebook - which has one billion users worldwide - would be moving into hardware with a smartphone or tablet product of its own.

Other commentators had suggested that Facebook would announce a charge for sending messages to users, an overhaul of its news feed feature or a new advertising network.

Zuckerberg said the new search tool would be rolled out "very slowly", starting with a beta version available to a limited number of users from today.

"This is one of the coolest things we have done in a while," he said, adding: "Graph Search is the kind of product that we love to build at Facebook - it's a big technology problem and it's also a big social problem.

"It's a kind of problem that Facebook and our culture are uniquely suited to build."

Although he repeatedly stressed the new service was different from a web search, Zuckerberg said Graph Search would be "integrated" with Microsoft's Bing search engine.

Searches that are not "mapped out" by Graph Search will be redirected to Bing, under an agreement between Facebook and Microsoft.

Asked whether this targeted Google, Zuckerberg said: "You know, I would love to work with Google - when we did our Bing web search integration we were very public about the fact this wasn't something we were trying to do with Bing.

"We want to make web search social in general."

Questions immediately arose over how the new tool will affect Facebook users' privacy.

But Facebook insisted that the feature included an opt-out option to keep user's content out of searches.

Chris Edwards, an electronics correspondent for Engineering and Technology magazine, said: "People don't really think about privacy and what they are uploading until they see stories about abuses.

"But if you look at some of the features available on Facebook, it's almost like a stalkers tool kit."

Asked whether Graph Search would be competing for market share with Google's search engine, Mr Edwards said: "Facebook really need people to comply for this new feature to work.

"The difficulty they face is that Google already provides easy access to information that people want to be found, whereas with Graph Search some users may want to conceal information."

Facebook went through what Zuckerberg described as a "rough cycle" after it became a public company amid wild expectation last May.

After starting at a per-share price of $38, the stock's performance sagged, but had mysteriously picked up in recent weeks as rumours spread about Tuesday's announcement.

The company will release its quarterly earnings figures on 30 January.

Zuckerberg first started Facemash in 2003 while he was a computer science and psychology student at Harvard.

By 2007, the site's evolution into the worldwide Facebook phenomenon had made the then 23-year-old one of the world's youngest ever self-made billionaires.